1. Field of the Invention
The present invention relates to an electrode used in an electrochemical reaction and a fuel cell using this electrode.
2. Description of the Related Art
The background of the present invention will be described with reference to a phosphoric acid fuel cell. In the phosphoric acid fuel cell, two electrodes, i.e., a flat cathode at which oxygen reacts and a flat anode at which hydrogen reacts oppose each other through an electrolyte layer. Each electrode has a two-layered structure consisting of a porous diffusion layer consisting of carbon fibers and having a thickness of about 1 to 2 mm and a porous catalyst layer formed on a surface thereof, which faces the electrolyte layer, and having a thickness of about 0.1 to 0.5 mm. A gas passage is formed in the diffusion layer or a separator adjacent to the diffusion layer. A reaction in the fuel cell takes place particularly in the catalyst layer.
When the cathode and anode of the phosphoric acid fuel cell are connected through an external circuit, reactions represented by formulas (1) and (2) below proceed in the cathode and anode, respectively. Hydrogen ions H.sup.+ and electrons e.sup.- produced from hydrogen H.sub.2 in the anode in accordance with formula (2) reach the cathode through the electrolyte layer and the external circuit, respectively, and react with oxygen O.sub.2 to produce water H.sub.2 O. In this case, a flow of electrons through the external circuit, i.e., a work of a current brings on energy extraction. EQU O.sub.2 +4H.sup.+ +4e.sup.- .fwdarw.2H.sub.2 O (1) EQU H.sub.2 .fwdarw.2H.sup.+ +2e.sup.- ( 2)
In these formulas, hydrogen ions H.sup.+ and electrons e.sup.- are present in an electrolyte and a solid substance, respectively. For this reason, the reaction field is in the solid/liquid interface. The reaction speed varies depending on the types of solid substances. To obtain a practical reaction speed, a catalyst mainly consisting of a noble metal is generally required. In the phosphoric acid fuel cell, platinum or an alloy containing platinum is very popular as the catalyst. To increase the reaction area per volume, platinum or an alloy containing platinum is used such that it is granulated into a fine powder whose particle has a size of several nm to several tens of nm, and the fine powder is dispersed and carried on the surfaces of carbon powder particles in practical applications. More specifically, the reaction field is the interface between the liquid electrolyte and the fine catalyst powder in the catalyst layer, and the catalyst must be wet with the liquid electrolyte.
O.sub.2 as an oxidant (reactant) appearing in formula (1) and H.sub.2 as a fuel (reactant) appearing in formula (2) are diffused as gases, dissolved in the liquid electrolyte, and diffused to the surface of the catalyst in the liquid. The diffusion rates of these substances in a liquid are much lower than those in a gas. For this reason, to attain a quick reaction, the diffusion distance in the liquid must be minimized.
To satisfy this requirement, there is proposed a method of mixing polytetrafluoroethylene (PTFE) powder, which is water-repellent, and a carbon powder, which carries a dispersed hydrophilic catalyst, and using the resultant mixture as a material constituting the catalyst layer. More specifically, after the catalyst-carrying carbon powder and the PTFE powder are stirred and mixed, the surface of the diffusion layer is coated or dusted with the resultant mixture. The resultant layer is worked with a roller. To improve the dispersion degree of the PTFE powder, the worked layer is heat-treated at a temperature of about 300.degree. C. to 390.degree. C., thereby forming a catalyst layer on the diffusion layer.
According to the concept of this method, although the surface of the carbon powder particles which carries the dispersed catalyst becomes wet with the liquid electrolyte, the oxidant and fuel in a gaseous phase penetrate into a region in which the PTFE powder particles are combined or very close to each other, so that the diffusion distance in the liquid can be shortened. For this reason, according to the method described above, to assure a gas passage in the catalyst layer, the PTFE powder particles must be connected or very close to each other. However, the connection or the like of the PTFE powder particles is a stochastic phenomenon because the carbon and PTFE powder particles are almost uniformly distributed by stirring. To increase the probability of connection or the like between the PTFE powder particles, PTFE must be used in a large amount. In this case, some of the catalyst-carrying carbon powder particles are surrounded and isolated by or covered with the PTFE powder particles to increase the resistance to electron conduction or ionic conduction or to decrease the surface area of the catalyst, which actually contributes to the reaction. It is, therefore, difficult to obtain a high output density in the catalyst layer formed by this method.
Jpn. pat. Appln. KOKOKU Publication No. 63-19979 discloses a gas-diffusing electrode material consisting of a porous structure which has entirely continuous fine pores. The structure comprises fine knots of a PTFE resin and a large number of fine fibers of the PTFE resin, which contain no conductive material powder particle and which extend from the respective knots and three-dimensionally couple the knots to each other. According to this structure, the fine knots are partially in contact with each other or are continuous with each other. In addition, an electrolyte and/or water hardly permeate a space constituted by only the fine fibers of the PTFE resin, thereby providing a gas diffusion passage therein.
In the structure proposed by No. 63-19979 set out above, however, the problems as partial elimination of the electrolyte necessary for the reaction, coverage of the catalyst surface with the PTFE particles, and degradation of electron conductivity cannot be solved because the PTFE volume inside the fine knots undesirably increases. In addition, the moving distances of ions, electrons, and gases increase due to the three-dimensional gas diffusion passage, thereby increasing the resistance.